The dentist says the filled molars are likely to crack. This will make the tooth unable to be crowned and would have to be pulled. To preempt this problem he wants to crown them now. I am 51 and the teeth seem in decent shape. I believe the dentist removed the old metal fillings and put these new white ones in only 3-5 years ago.

Note: The dentist wanted me to check my insurance for crowns... but he has been my dentist for 7 years and knows my insurance pays 90%. But this would be my first crown from him.

Is this common practice, to cap old (recently) filled teeth ?

This is a very common treatment recommendation. You should still have a predetermination sent to the insurance. That way you will know the exact dollar amount you will be responsible for PLUS the insurance company may have criteria that must be met in order to pay for a crown. In the years I've been working in dental most insurance companies will want an x-ray and/or photo of the tooth before treatment is started. 85% of insurance companies here will only pay for a crown if one of the tooths cusps are no longer present.

You don't want to have crowns placed thinking they will be covered at 90% only to find out after that the condition of the tooth did not meet the insurance companies criteria for crowns, or you have exceeded your allowable amount for crowns or other dental work.

What you are telling me is that it is common practice to crown teeth whose only problem is that the fillings are large. He showed me pictures of the 3 teeth he wants to crown, two appeared in perfect condition, the third had only a hairline of discoloration next to the inlaid filling.
Maybe the insurance companies are trying to stop this scam by saying that the cusps or some other problem must be present... this seems reasonable to me.

As an engineer, I was wondering if there is data on the strength of a tooth as someone ages. Or statistics on the number of molars that do crack due to inlaid fillings. This way I could weigh the risk myself.

With the x-rays of my previous (3-5 years ago) metal fillings, the dentist could see how large they were. Why didn't he suggest crowns at this time?

Teeth with large fillings in them ARE at increased risk of fracturing. If the teeth that need crowns are teeth that you use to do most of your chewing on then it crowns are often recommended. The fillings should cover more than just one tooth surface though before a crown is recommended. If the filling is just on the biting surface it's not at as great a risk to fracture.

Silver fillings expand and contract with the temperature of the foods and beverages you consume and you can actually start to see cracks forming in the teeth over time. The life span of a silver filling is between 3-8 years.

It seems your dentist wants to be more proactive and prevent the chance of the filling/tooth breaking by placing crowns.